In the electric utility industry, it is common practice to "preheat" air which is being input to a boiler, or steam generator used in conjunction with a turbine-generator for generating electrical power. By preheating input air used for combustion, the amount of fuel required to produce a certain amount of energy is reduced and thus boiler efficiency is improved.
The input air is heated through a type of heat exchanger known in the industry as an air preheater. Exhaust gases, containing heat energy emitted from the boiler, are used to heat a number of heat exchange elements which are configured in the shape of a porous wheel and are commonly referred to as a basket. This basket may be as large as 50 to 60 feet in diameter and 10 feet thick and may be oriented either vertically, or horizontally.
The exhaust gases are directed, via an exhaust duct, to flow through approximately one-half of the basket, thus transferring the heat energy from the exhaust gas to a specific area of the basket. Simultaneously, input air flows via an input air duct through the other half of the basket. Transfer of the heat energy from the basket to the input air occurs as the basket is rotated, wherein the heated portion passes through the input air flow and thus the input air is heated up.
Two types of air preheaters are widely used, the first being a Ljungstrom as described above. The second is a Rothmuhle which operates in a similar fashion to the Ljungstrom, however the basket remains stationary and the air input duct is rotated about a bearing. Therefore, the input air is heated as it flows through the basket, which has obtained the heat energy from the exhaust gas.
In electric power plants, particularly those using coal for fuel, the exhaust gas from the boiler contains large quantities of particulate matter, specifically fly ash and soot which tend to clog the basket after continued use. This clogging of the basket restricts the flow of the input air and the exhaust gas, thus causing a significant drop in efficiency.
Cleaning the air preheater basket has been a major problem and a significant cause of costly down time throughout the electric utility industry. Previous cleaning methods include using fire hoses and air pressure to remove fly ash from the baskets, but these methods have only met with partial success. Alternatively, replacing the baskets has been another solution to the low efficiency problem caused by a clogged air preheater basket, however this is extremely costly in terms of down time and expense.
An air preheater cleaning apparatus is disclosed by U.S. Pat. No. 4,256,511 to Atchison, et al, which provides an automatic jet wash system for cleaning the air preheater baskets. Atchison et al states that the air preheater cleaning system can be used to clean air preheaters of other than the Ljungstrom type, however it does not seem possible to use the Atchison et al system on a Rothmuhle air preheater wherein the air input duct is rotated with respect to a stationary basket. For example, no pivot or swivel assembly is provided which would prevent the microswitch lead and air supply line from becoming tangled upon rotation of the air preheater cleaning apparatus. Another disadvantage of Atchison et al is that the basket is rotated at a constant rate of speed, unless manually increased, and as the water jet moves inward, the speed of the rotating basket with respect to the water jet slows down. Therefore, costly down time is unnecessarily used for cleaning, where if the rotational speed of the basket could be increased as the jet moves inward, down time could be reduced.
Another automatic air preheater cleaning system is manufactured by Weatherford Water Jetting Systems. The Weatherford cleaner does increase the rotational speed of the basket during cleaning, but the system contains several other drawbacks. Particularly, as in Atchison et al, the Weatherford system cannot be used on Rothmuhle type air preheaters and an electric motor is used to drive the jet head assembly. Moreover, the control system in the Weatherford cleaner uses a clock and an assumed basket speed in order to calculate the rotational basket speed increases which take place as the jet assembly moves inward, and further the control system does not provide job interrupt and reset functions. Another disadvantage of the Weatherford system is the fact that only a single jet head nozzle is utilized, thus limiting the amount of basket surface which can be cleaned per revolution.
In view of the foregoing disadvantages and drawbacks in the art, an air preheater cleaning system such as the present invention which can be used on both Ljungstrom and Rothmuhle type air preheaters and in conjunction with variable speed control apparatus is highly desirable. Further, the present invention utilizes a totally pneumatic water jet drive and logic system, provides interrupt and reset functions and uses a motion sensing device for determining actual position values, rather than using assumed values. Therefore, a greater amount of flexibility and control over an air preheater cleaning job is provided by the present invention and the amount of down time experienced by the electric utility is held to a minimum.